Our objective is to elucidate how cells derived from the mammalian brain perceive and respond to the signals in their environment. We have studied the effects of hormones and neurotransmitters which transmit information across the plasma membrane while binding to and remaining at cell-surface receptors of neurons and astrocytes. In rat astrocytoma cells, we have shown that certain neurotransmitter-stimulated, receptor-mediated increases in cyclic AMP synthesis are modulated by tricyclic antidepressants such as imipramine after the drugs are transported into cells and accumulated in subcellular organelles. We have shown that growth of astrocytoma cells in the presence of imipramine causes down-regulation of beta-adrenergic receptors, a phenomenon which mimics the clinical response to long-term administration of imipramine. Our results suggest that imipramine may cause down-regulation of these receptors by accumulating in endocytic vesicles and impairing receptor recycling. Using primary cultures highly enriched for a particular type of neuron, we have found that the ability to respond to stimulation of cyclic GMP production by excitatory amino acids is related to the developmental stage of the neurons. The stimulation of cGMP production correlates with the stimulation of neurotransmitter release from these neurons. Furthermore, stimulation of cyclic GMP production by excitatory amino acids and by depolarizing agents is calcium dependent and is inhibited by imipramine. Our results implicate calcium in the activation of guanylate cyclase, the biosynthetic enzyme for cyclic GMP, and suggest a biochemical basis for the manifold actions of tricyclic antidepressants. We have found that imipramine and related compounds are also concentrated in intracellular vesicles in cultured neurons, and we are presently investigating the possible role of tricyclic antidepressants in the depletion of neurotransmitters in secretory vesicles.